1. Field of Invention
The present invention relates to a method of measuring electrical capacitance in which a scanning capacitance microscope is used to measure electrical capacitance of semiconductor surface(s).
2. Conventional Art
Scanning probe microscopesxe2x80x94scanning tunneling microscopes and atomic force microscopes being representative thereofxe2x80x94are effective as measuring apparatuses in evaluation of small domains such as semiconductor sample surfaces and the like.
Atomic force microscopes are apparatuses which have made it possible to acquire information about surface roughness of samples through detection of atomic forces acting between the atoms at a sample surface and a probe at the end of a cantilever, and are effective analytical apparatuses for obtaining information in the small domains characteristic of semiconductor sample surfaces (see, e.g., Japanese Patent Application Publication Kokai No. H8-189931 (1996)).
Atomic force, being a parameter which is detected by atomic force microscopes, can be determined based on optical detection of cantilever deflection. Moreover, because cantilever deflection and atomic force are proportional, by measuring atomic force based on cantilever deflection and representing this as a two-dimensional image it is possible to acquire information about surface roughness and the like.
One technique making use of the atomic force microscope is scanning capacitance microscopy. A scanning capacitance microscope, being an apparatus which is capable of measuring the two-dimensional profile of impurity carriers at a semiconductor surface, is an extremely effective analytical apparatus for use in the development of various microelectronic devices.
A scanning capacitance microscope, for example as shown in FIG. 3, may comprise electrically conductive probe 101, movable stage 102 employing piezoelectric elements, capacitance measuring apparatus 103, and so forth; the apparatus employing capacitance measuring apparatus 103 to measure electrical capacitance between sample S and electrically conductive probe 101 as a result of application of an AC bias voltage between sample S and electrically conductive probe 101 while electrically conductive probe 101 is in contact with sample S, and moreover measuring the distribution in electrical capacitance across the surface of sample S as a result of two-dimensional movement of sample S by movable stage 102, so as to permit a two-dimensional profile of impurity carriers at the semiconductor surface to be obtained from the results of those measurements. Note also that natural oxide film 100 is formed on the surface of sample S; this will be described below.
Below, the principles behind measurement using scanning capacitance microscopy are described.
First, when the region doped with impurity carriers is scanned during scanning of a sample surface with a probe, a MOS-like structure will be formed beneath the probe because the probe, an insulating film, and impurity carriers are located in a sequential order. At such time, a depletion layer forms between insulating film and impurity carriers, application of an AC bias voltage to this depletion layer causing a change in capacitance across the depletion layer. Because the magnitude of this change in capacitance is dependent upon impurity carrier concentration, graphic representation of the change in capacitance makes it possible to measure the two-dimensional carrier profile at the region being scanned on the semiconductor surface. Moreover, during such measurement using scanning capacitance microscopy, conventionally, a natural oxide film which forms on the surface of the sample being measured has typically been used as the aforesaid insulating film.
Furthermore, when scanning capacitance microscopy is used to measure the capacitance distribution of a semiconductor surface, the natural oxide film which forms on the surface of the semiconductor sample has, as has been stated above, ordinarily been used as the insulating film necessary for measurement of electrical capacitance. But because natural oxide films are extremely unstable, with formation of complex surface states at interfaces and occurrence of surface defects and the like, there is the problem that stable, accurate measurement of electrical capacitance of semiconductor surfaces has not been permitted and it has not been possible to obtain data with good reproducibility.
The present invention was conceived in order to solve such problems, it being an object thereof to provide a method of measuring electrical capacitance permitting stable, accurate measurement of electrical capacitance of semiconductor surfaces using scanning capacitance microscopy.
A method of measuring electrical capacitance in accordance with an embodiment of the present invention is such that a scanning capacitance microscopes is provided for detecting at least one surface by means of at least one electrically conductive probe used to measure electrical capacitances of semiconductor surfaces. The method comprising a first steps wherein at least one clean surface is formed on at least one semiconductor samples by surface treatment. In a second step, the at least one semiconductor sample on which at least one clean surfaces was formed is promptly (preferably within 10 minutes) placed in an ultrahigh vacuum environments (e.g., an ultrahigh vacuum environment of on the order of 1.33xc3x9710xe2x88x927 Pa (1xc3x9710 Torr)) or in an inert gas environments and is maintained therein. In a third step, at least one electrically conductive probes, on a surface of which an insulating films is formed, is used to measure the electrical capacitances of the at least one semiconductor sample surface maintained in the ultrahigh vacuum environments or in the inert gas environment.
In another embodiment, the method of measuring an electrical capacitance is such that a plurality of scanning capacitance microscopes are provided for detecting a plurality of surfaces by means of each microscope having an electrically conductive probe used to measure electrical capacitances of semiconductor surfaces. The method comprising a plurality of first steps, i.e., a first step for each individual microscope, wherein at least one clean surface is formed on semiconductor samples by surface treatment. In a plurality of second steps, the semiconductor samples on which at least one clean surface was formed are promptly (preferably within 10 minutes) placed in at least one ultrahigh vacuum environment (e.g., an ultrahigh vacuum environment of on the order of 1.33xc3x9710xe2x88x927 Pa (1xc3x9710xe2x88x929 Torr)) or in at least one inert gas environment and are maintained therein. In a plurality of third steps, a plurality of electrically conductive probes, i.e., one for each microscope, on each surface of which an insulating film is formed, are used to measure the electrical capacitances of the at least one of the semiconductor sample surface maintained in the at least one ultrahigh vacuum environment or in the at least one inert gas environment.
In accordance with one or more embodiments of the present invention, because measurement is carried out not using unstable natural oxide film(s) which form on semiconductor sample surface(s) but using stable insulating film(s) formed by vapor deposition or the like on surface(s) of electrically conductive probe(s) as insulating film(s) necessary for measurement of electrical capacitance of semiconductor surface(s), stable and accurate measurement of electrical capacitance is permitted. Moreover, because measurement is carried out in ultrahigh vacuum environment(s) or inert gas environment(s), oxide film(s) whose growth might otherwise be promoted by electric field(s) or the like during measurement do not form substantially on semiconductor sample surface(s).
Note that it is preferred in accordance with the present invention that insulating film(s) formed on surface(s) of electrically conductive probe(s) be vapor-deposited film(s) of wear-resistant insulating material(s) of sufficient hardness, e.g., insulating diamond, DLC (diamond-like carbon), alumina, and/or zirconium oxide, and that deposited film thickness be not more than 5 nm.